Welded Contact on a Star Delta 1

[SparkyLarks]

With a 3-phase star Delta motor , with three contactors.

Does anybody know what will happen if one of the contactors welds shut.

Essentially there will be a voltage supplied to one side of the windings.

I am having a discussion whether the contactors on both sides of the winding is sufficent for a redundant shutdown( this is for a safety EN 13849-1 applications)

If once contact welds, I may have 230V AC on each side of the winding, but that is 0V Across it.

My own view, IS that I would prefer to have an extra contact, that will remove the voltage from each side of the winding, but this arrangements can be considered redundant.

 

[jraef]

Depending on which contactor welds, you can have anything from power on one side of the motor windings at all times, to a bolted fault (line-to-line short circuit) that could be destructive to the starter and other periferal equipment. But if it is the 1M (K1) contactor that welds, this is actually something that is very commonly left off of OEM equipment anyway, it is only there to provide isolation in the first place. Whether or not to include it is up to you, but another redundant contactor seems excessive to me.

If you are concerned about this, why are you using Star-Delta at all? If a welded contactor were a critical concern, I would get rid of the Star-Delta scheme (since it is very prone to such occurrances) and use a Solid State Starter with an isolation contactor. The chances of that contactor ever welding are astronomically small but it provides the added safety of protection against a shorted SCR.

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[SparkyLarks]

why am I using Star Delta? I'm not.I would use a VFD personally but I am reviewing a design.
They key question at hand is does the use of the K1,K2,K3( line Star and Delta) contactors, mwwt the requirements of EN ISO 13849-1. Typically Machinery builders will take the cheapest approach.

As this functin requires a redundant shutdown of the morot, I reccomended a 4th contactor which would cut power to both the line and star.Delta side. But machine builders typically do not want to do this.

to meet that I require redundancy, and I have talked to some experts in this area and they have differing views. The most common one is to say that it does not comply, but when chalenged to explain why then they cannot give a reason.

the key requirement is that a single fault will not cause a failure of the safety functions. To consider the case wher the line contactor welds but the power Star and delta open, then does the voltage not present at the winding constitut a failure of the safety function? Well the motor will not move , unless a current path is present, and an earth fault could supply this, though that is a second fault.

 

[LionelHutz]

I would believe the wye-delta starter scheme would need to experience 2 failures to make the motor shaft move when it should not. Does that not meet the safety requirements?

 

[mayisee]

Normal conditions...Main contactor closes at start and remains closed whilst motor is running. The star contactor closes with the main at start (star and the delta are electrically interlocked with main contactor IE main must be closed)and is timed (nominally 10 secs)to open and when open allows delta contactor to close and remain closed while motor is running.(the star and delta contactors are electrically and mechanically interlocked so only one can be closed)Contactor seizure is in the worst condition when the main contactor seizes at start or whilst running.This would mean that the motor could not be stopped by a stop button or overload. The seizure of star whilst motor is starting could give a condition where the motor will be left running in star and possibly go into overload conditions depending on mechanical load. A delta seizure whilst running may not show up until motor is stopped and restarted. Both star and delta seizure will give a condition where the motor will not start at the next attempt due to electrical interlock with main.Note these conditions will only be true if the appropriate interlocks are wired into the control of the motor.

 

[LionelHutz]

Contactor seizure is in the worst condition when the main contactor seizes at start or whilst running.This would mean that the motor could not be stopped by a stop button or overload.

You can build a control wiring scheme that would allow the motor to be stopped by the wye and delta contactors even if the main contactor welds.

 

[mayisee]

Yes you are correct Lionel, by wiring in a n/c stop button contact into both star and delta contactors.This is the benefit of have three contactors in a star/delta system, but where a standard DOL motor contactor seizes it becomes more difficult and dangerous to stop the motor.

 

[ScottyUK]

Is contact welding a common failure mechanism for NEMA contactors? IEC contactors, when protected by the correct fuse or breaker, are specifically not allowed to weld under the standard tests in IEC 60947. I've only seen contacts weld a couple of times, once because the contactor armature didn't make its full travel resulting in inadequate contact pressure, and the other when the plastic of the cheap expensive & nasty contactor melted and caused the moving poles to move away from the fixed poles. It's slightly un-nerving when the motor won't stop in response to an e-stop!